The present invention relates generally to electroslag refining, and, more specifically, to electroslag refining of superalloys.
Electroslag refining is a process used to melt and refine a wide range of alloys for removing various impurities therefrom. U.S. Pat. No. 5,160,532-Benz et al. discloses a basic electroslag refining apparatus over which the present invention is an improvement. Typical superalloys which may be effectively refined using electroslag refining include those based on nickel, cobalt, zirconium, titanium, or iron. The initial, unrefined alloys are typically provided in the form of an ingot which has various defects or impurities which are desired to be removed during the refining process to enhance metallurgical properties thereof including grain size and microstructure, for example.
In a conventional electroslag apparatus, the ingot is connected to a power supply and defines an electrode which is suitably suspended in a water cooled crucible containing a suitable slag corresponding with the specific alloy being refined. The slag is heated by passing an electric current from the electrode through the slag into the crucible, and is maintained at a suitable high temperature for melting the lower end of the ingot electrode. As the electrode melts, a refining action takes place with oxide inclusions in the ingot melt being exposed to the liquid slag and dissolved therein. Droplets of the ingot melt, fall through the slag by gravity and are collected in a liquid melt pool at the bottom of the crucible. The slag, therefore, effectively removes various impurities from the melt to effect the refining thereof.
The refined melt may be extracted from the crucible by a conventional segmented, cold-walled induction-heated guide (CIG). The refined melt extracted from the crucible in this manner provides an ideal liquid metal source for various solidification processes including, for example, powder atomization, spray deposition, investment casting, melt-spinning, strip casting, and slab casting.
In the exemplary electroslag apparatus introduced above, the crucible is conventionally water-cooled to form a solid slag skull on the surface thereof for bounding the liquid slag and preventing damage to the crucible itself as well as preventing contamination of the ingot melt from contact with the parent material of the crucible, which is typically copper. The bottom of the crucible typically includes a water-cooled, copper cold hearth against which a solid skull of the refined melt forms for maintaining the purity of the collected melt at the bottom of the crucible. The CIG discharge guide tube or downspout below the hearth is also typically made of copper and is segmented and water-cooled for also allowing the formation of a solid skull of the refined melt for maintaining the purity of the melt as it is extracted from the crucible.
A plurality of water-cooled induction heating electrical conduits surround the guide tube for inductively heating the melt for controlling the discharge flow rate of the melt through the tube. In this way, the thickness of the skull formed around the discharge orifice in the guide tube may be controlled and suitably matched with melting of the ingot for obtaining a substantially steady state production of refined melt which is drained by gravity through the guide tube.
The cold hearth and the guide tube of the conventional electroslag refining apparatus are relatively complex in structure, and are therefore expensive to manufacture. The guide tube typically joins the cold hearth in a conical funnel configuration, with the induction heating coils surrounding the outer surface of the funnel and the downspout through which the melt is drained from the crucible. Furthermore, each of the guide tube segments or fingers must also be suitably manufactured with internal cooling passages therein which adds to the complexity of the assembly and cost of manufacture.
The funnel-shaped guide tube is also subjected to substantial stress and strain during operation from its complex three-dimensional configuration and from the heating and cooling effects of the melt, coolant, and induction heating. The useful life of the guide tube is therefore limited, and repair and replacement thereof requires the disassembly of all components in the vicinity thereof to provide access thereto which results in a substantial down-time during a maintenance outage. And, the funnel-shaped guide tube requires complex manufacturing processes to build including specialty milling of the various components and fabrication and assembly thereof.
It is therefore desirable to reduce the complexity of the guide tube and adjoining cold hearth for reducing the cost of manufacture, and improving the assembly and disassembly thereof.